Duplex metal article



DUPLEX METAL ARTICLE Robert B. Mears, Oakmont, Pa., assignor to United States Steel Corporation, a corporation of New Jersey Application February 13, 1952,. Serial No. 271,329

2 Claims. (Cl. 29--196.1)

This invention relates to duplex metal articles and in particular to a structural material comprised of a stainless steel clad with a layer of stainless steel of lower chromium content.

Stainless steel alloys containing more than about 16% chromium either with or without nickel and other alloy- 20 ing elements are extremely resistant to corrosion under most conditions. However, when exposed to chloride solutions, e. g., sea water, the-se alloys are subject to severe localized pitting attack. This susceptibility to putting in chloride solutions is well known to those who have studied the corrosion of stainless steels. The pitting p is especially likely to occur at edges, sharp corners, and lat areas of the surface which are wholly or partially shielded from oxygen, i. e., the areas of contact at overlapping joints and the spots beneath barnacles or other solid materials in intimate con-tact with the surface.

Except for this localized attack, the overall corrosion of `stainless steels in sea water is negligible. However, the rate of corrosion at pits is very rapid and perforations, necessitating costly repairs, soon develop. Perforation at critical points has resulted in failure of entire structures even though only isolated areas lthereof were aifected by the corrosion. The tendency to localized attack, also increases the susceptibility to stress-corrosion cracking and failure from this cause often occurs before perforation is complete.

The above behavior is characteristic of the stainless steels and has prevented their utilization in marine stil-ls, condensers, channel-buoys 4and similar structures, uses for which they are otherwise ideally suited.

'It is an object of the present invention to provide a new structural material possessing the overall corrosion resistance of stainless steel plus resistance to perforation when exposed in chloride solutions such as sea water and the like.

Another object is to provide a stainless steel article which is more resistant to stress-corrosion cracking than such articles available heretofore.

These and other objects will become apparent from the following spec-ication when read in conjunction with the att-ached drawings in which:

Figure l shows one form of my invention, adapted to service in which only one surface of the article is subjected to the chloride salt solutions; and

Figure 2 shows an alternate form adapted to service wherein both surfaces of the article will be in contact with the corroding medium, as for example, the tubes of a marine still.

As indicated in the drawings the present invention contemplates combining layers of stainless steels of certain specific chromium contents. In general, I have discovered that when a layer 1 of `a stainless steel containing less than 14% chromium is superimposed on a layer Z of stainless steel containing more than 16% chromium, the resulting duplex metal possesses excellent overall corrosion resistance and does n-ot perforate when exposed to solutions containing chloride salts. Any such pitting as may occur is confined to the llow chromium layer and furthermore the progress of the pits is arrested at the interface of layers 1 and 2, so that the pits do not become stress-raisers, consequently stress-corrosion cracking is completely avoided.

To achieve these results the layer 2 must contain not less than 16% chromium and the chromium content of the layer 1 must be sufliciently less than 16%A so as tol establish this layer as at least 0.1 volt more anodic than layer 2 when immersed in the chloride solution. To achieve this necessary minimum difference in solution potential imposes a maximum of chromium content on layer 1. While this varies somewhat with the composition of the corroding medium, it has been found that layer 1 should never contain more than 14% chromium. Decreasing the chromium content of layer 1 from the aforementioned maximum increases the difference in solution potential of the layers, however, the overall-corrosion resistance of layer 1 also decreases with decreasing chromium content and the layer must contain not less than 4% chromium for this reason. For most conditions of service a chromium content of layer 1 of about 12% has 'been found to afford an optimum balance between protective effect and overal-l corrosion resistance.

Increasing the chromium content of layer 2 has negligible effectv on difference in solution potential of the layers. However chromium contents higher than 16% `are often necessary to achieve other mechanical properties in the duplex. For this reason it is contemplated that layer 2 may con-tain up to 30% chromium as well as quantities of Ni, Mo, Cu and other a'lloying elements added ,for their known effect.

The method of combining the layers 1 and 2 forms no part of the present invention. Any of several known methods may be used. However, the process of cladding in which metal .sl-abs of the required analyses are heated to welding temperature and rolled together to bond the `slabs together `and form a duplex metal is particularly adapted t-o my purposes.

As an example of the practices and results of the present invention, a slab of AiSl type 304 austenitic stainless steel maybe clad on both sides with AISI type 410 and rolled to about 1A inch plate having a central `section containing about 18% chromium, .about 0.168 inch thick 4and outer layers containing 12% chromium, about 0.026 inch thick. The central, 18% chromium, layer of this duplex is unpitted, even at the cut (unclad) edges thereof, after 1100 hours of exposure to aqueous solutions containing as much as 3% sodium chloride plus 2.0% hydrogen peroxide. Some pitting of the 12% chromium layers occurs but the pits terminate lat the interfaces of the 10W and high chromium layers. In contrast, both of the materials of the duplex are deeply pitted and perforated when exposed separately to the above conditions. Similar results are obtained by cladding AISI type 430 with type 410; and by cladding AISI types 310 and 446 with the 12% chromium alloy, clearly indica-ting that the results are dependent on etfects arising from the difference in chromium content of the layers and are relatively independent of the nickel and/or other alloy content thereof.

While the materials in the foregoing examples are commercial alloys, such have 'been chosen for convenience in describing the invention and the analyses` of the layers need not be limited thereto except as previously set forth. The duplex metals of the examples are particularly adapted to service in sea water wherein a cladding of 12% chromium alloy has been found to provide an optimum balance of protection against pitting and overall corrosion resistance. The latter is of particular importance in the construction of, for example, marine stills and the like wherein contamination of the water or other liquid being treated with ferrous oxides must be avoided. However, as previously mentioned, the cladding alloy must be at least 0.1 volt more anodic than the base or core section, `and since the solution potentials vary to some degree with the nature and concentration of the chloride salts of the corroding medium, it is preferable, in the general utilization of my invention, to measure the halfcell potentials of Various chromium alloys in the particular solution to be encountered in service, and then select an alloy for the cladding material having sufficiently low chromium content to provide at least the required minimum potential difference. A potential diiference of 0.2-0.3 volt anodic is preferred and larger differences should be avoided since the chromium content of the cladding and therefore its overall corrosion resistance is unduly lowered. Moreover larger differences are unnecessary, e. g., type 410 (12% chromium) stainless steel is 0.19 to 0.41 volt anodic to type 304 (18% chromium-8% nickel) stainless steel in an aqueous solution of 3% NaCl and 2% H2O2 yet when type 304 stainless steel is clad on one side only with type 410 in accord-ance with this invention, I have found the unclad side remains unpitted when exposed to sea water and other chloride solutions.

The fact that such remote areas of the high chromium layer are protected also indicates .that thickness of the cladding is not critical insofar as the initial protective effect is concerned. Thickness of the cladding, however, governs the permanency of the protection. Thicknesses of the order of 0.02 inch are recommended for most services. I1n some instances, of course, thinner claddings will give satisfactory service life, While in others, Where long life is the basic consideration, `a much heavier cladding will be desirable. In all instances the thickness of the base metal (high chromium layer) is dictated by the strength requirements of the article or structure for while the cladding, being itself a structural material, will conlamargos fore provide an accelerated test of the materials. The fact that duplex metals of my invention do not perforate in 1100 hours exposure to such solutions indicates a service life of many years in sea water or the like. IFurther,

-while my invention and results are described as respects chloride solutions it will be evident to those skilled in the art that similar results will be obtained in aqueous solutions of the other halogen salts, the behavior of the chromium stainless steels in such media being quite similar.

I claim:

1.*In manufactures of the class described, laminated structures composed of layers of stainless steel, which manufactures are highly resistant to perforation and stress corrosion cracking when exposed to halogen salt containing solutions and comprise a base layer of stainless 'steel containing between 16 and 30% chromium and a facing layer of stainless steel bonded thereto and adapted -t-o be directly exposed to the halogen salt containing solution, said facing layer containing about 12% chromium and having `a solution potential at least 0.1 volt more anodic 4than the base layer when immersed in the halogen salt containing solution.

2. In manufactures of the class described, laminated structures composed of layers ot" stainless steel, which manufactures are highly resist-ant to perforation and stress corrosion cracking -when exposed to halogen salt containing solutions and comprise a base layer of stainless steel containing not less than 16% chromium and al facing .layer of stainless steel bonded thereto and adapted to be directly exposed to the halogen salt containing solution, -said facing layer containing at least 2% less chromium ""2 than the base layer and having a solution potential at tribute to the strength of the article, this contribution will The peroxide-containing sodium-chloride solutions referred to in the foregoing examples are by way of illustration only. Such solutions are considerably more corrosive than ordinary brines and/ or sea water, and thereleast 0.1 volt but not more than about 0.4 volt more anodic than the base layer when immersed in said halogen salt containing solution.

References Cited in the le of this patent UNITED STATES PATENTS 1,608,694 Cain Nov, 30, 1.926 1,929,655 Scott Oct. l0, 1933 1,998,496 Fiedler Apr. 23, 1935 2,034,278 Becket Mar. 17, 1936 2,249,629 Hopkins July 15, 1941 2,306,421 Arness Dec. 29, 1942 2,544,335 Dinnert Mar. 6, 1951. 

1. IN MANUFACTURES OF THE CLASS DESCRIBED, LAMINATED STRUCTURES COMPOSED OF LAYERS OF STAINLESS STEEL, WHICH MANUFACTURES ARE HIGHLY RESISTANT TO PERFORATION AND STRESS CORROSION CRACKING WHEN EXPOSED TO HALOGEN SALT CONTAINING SOLUTIONS AND COMPRISE A BASE LAYER OF STAINLESS STEEL CONTAINING BETWEEN 16 AND 30% CHROMIUM AND A FACING LAYER OF STAINLESS STEEL BONDED THERETO AND ADAPTED TO BE DIRECTLY EXPOSED TO THE HALOGEN SALT CONTAINING SOLUTION, SAID FACING LAYER CONTAINING ABOUT 12% CHROMIUM AND HAVING A SOLUTION POTENTIAL AT LEAST 0.1 VOLT MORE, ANODIC THAN THE BASE LAYER WHEN IMMERSED IN THE HALOGEN SALT CONTAINING SOLUTION. 